On the very first day, God created the cow. He said to the cow, "Today I have created you! As a cow, you must go to the field with the farmer all day long. You will work all day under the sun. I will give you a lifespan of 50 years."
The cow objected, "What? This kind of tough life you want me to live for 50 years? Let me have 20 years, and the other 30 years I'll give back to you." So God agreed.